1. Field of the Invention
The present invention relates to a process for heterogeneously catalyzed partial gas phase oxidation of propylene to acrylic acid, in which, in a first reaction zone, a starting reaction gas mixture 1 which comprises propylene, molecular oxygen and at least one inert diluent gas and comprises the molecular oxygen and the propylene in a molar O2:C3H6 ratio of ≧1, in a first reaction stage at elevated temperature, is first conducted through at least one first catalyst bed whose catalysts have at least one multimetal oxide comprising Mo, Fe and Bi as the active composition, such that the propylene conversion in single pass through the catalyst bed is ≧90 mol % and the accompanying selectivity SAC of acrolein formation and of acrylic acid by-product formation together is ≧80 mol %, the temperature of the product gas mixture 1 leaving the first reaction stage is, if appropriate, reduced by direct cooling or by indirect cooling or by direct and indirect cooling, and, if appropriate, secondary gas in the form of molecular oxygen or inert gas or molecular oxygen and inert gas is added to product gas mixture 1, and then product gas mixture 1, as a starting reaction gas mixture 2 which comprises acrolein, molecular oxygen and at least one inert diluent gas and comprises the molecular oxygen and the acrolein in a molar O2:C3H4O ratio of ≧0.5, in a second reaction stage at elevated temperature, is conducted through at least one second catalyst bed whose catalysts have at least one multimetal oxide comprising Mo and V as the active composition, such that the acrolein conversion in single pass through the catalyst bed is ≧90 mol % and the selectivity SAA of acrylic acid formation assessed over both reaction stages, based on propylene converted, is ≧70 mol %, then the acrylic acid present in the product gas mixture 2 formed in the second reaction stage, in a first separation zone, is converted therefrom to the condensed phase and then, in a second separation zone, the acrylic acid is removed from the condensed phase by use of at least one thermal separation process.
2. Description of the Background
As a partial oxidation product of propylene, acrylic acid is a significant monomer which finds use, as such or in the form of its alkyl esters, for obtaining polymers suitable, for example, as adhesives or water-superabsorbing polymers (cf. for example, WO 02/055469 and WO 03/078378).
The preparation of acrylic acid by a process described at the outset of this document is known (cf., for example, DE-A 102 45 585, WO 03/011804, DE-A 101 31 297, WO 01/96270).
The propylene required as a starting substance for this procedure is adding to starting reaction gas mixture 1 typically as crude propylene. In contrast to chemically pure propylene, crude propylene also comprises constituents chemically different from propylene (impurities) which, based on the crude propylene, may be up to 10% by volume and more. For example, the crude propylene may also be the product gas mixture of a heterogeneously catalyzed partial propane dehydrogenation (cf. DE-A 102 45 585 and
DE-A 10 2005 022 798). In principle, it is possible to remove all impurities present in crude propylene from the propylene present therein (cf., for example, DE-A 35 21 458 and DE-A 102 45 585). However, this is not necessary when the impurities behave inertly in the heterogeneously catalyzed partial oxidation of propylene to acrylic acid. When they have the latter property, the impurities act simply as inert diluent gases in starting reaction gas mixture 1 (cf. WO 01/96270 and DE-A 33 13 573). In this document, this refers quite generally to those gases which, in the course of the partial oxidation, each alone, remain chemically unchanged to an extent of at least 95 mol %, preferably to an extent of at least 97 mol % and most preferably to an extent of 99 mol % or more. In the course of the conversion of the acrylic acid from product gas mixture 2 into the condensed phase, these inert gases typically remain as residual gas in the gas phase and can thus be removed from the target product in a comparatively simple manner after the partial oxidation than would be the case in a removal of propylene prior to the partial oxidation. In relation to the partial oxidation of propylene to acrylic acid, the propanes have hitherto been considered to be such inert diluent gases in the technical literature. The considerations in this connection even go to the extent of replacing propylene as a raw material for preparing acrylic acid by propane as such a raw material. In this case, propane is dehydrogenated partially to propylene in a first step and the propylene formed in the first step is subsequently partially oxidized to acrylic acid under heterogeneous catalysis in the presence of the unconverted propane (cf. WO 01/96270). Normally, propane in such a resulting starting reaction gas mixture 1 even formed the main constituent. Recycling of the residual gas which comprises unconverted propane and remains in the condensation of the target product out of the product gas mixture into the dehydrogenation and/or partial oxidation allows the propane in this way finally to be converted fully to acrylic acid (cf., for example, DE-A 02 45 585, DE-A 10 2005 009 885, DE-A 10 2005 010 111). Although a vanishingly small amount of the propane (its order of magnitude based on its use amount is 0.01% by weight) can be converted to propionic acid (which is an undesired companion to acrylic acid merely owing to its unpleasant odor even in the smallest amounts and owing to its incapability of polymerizing in a free-radical manner), such a small by-product conversion can be counteracted, for example, by diluting the starting reaction gas mixture 1 comprising propane additionally with an inert diluent gas other than propane (e.g. N2, H2O, CO2, noble gas, mixtures of these gases, etc.) (cf., for example, WO 01/96270).
However, the above-described considerations are no longer valid when the propylene contamination does not behave inertly in a heterogeneously catalyzed partial oxidation thereof to acrylic acid, but is instead converted in significant fractions to a by-product of acrylic acid formation. This is attributable to the fact that the by-product formed normally cannot be discharged as a target product impurity with the target product. Instead, in many cases, even small target product contaminations, with a view to the desired target product use, have a troublesome effect (for example, in the case of use of the acrylic acid to prepare polyacrylic acids and/or their fully and/or partly neutralized alkali metal salts, which are used predominantly as water-superabsorbing materials in the hygiene sector; or in the case of use of the acrylic acid for preparing its alkyl esters and the use of the latter for preparing polymers suitable as adhesives) and then have to be removed in a second separation zone using at least one thermal separation process from the phase which comprises the target product in condensed form and is formed in the first separation zone (or vice versa). Such a removal can be comparatively costly and inconvenient. In such cases, attempts will appropriately be made to remove the appropriate propylene impurity prior to the partial oxidation.
In many cases, a parallel procedure is also used. In other words, a portion of the propylene impurity is removed prior to the propylene partial oxidation, and the remaining portion is, after the propylene partial oxidation has been carried out, removed from the acrylic acid as an acrylic acid by-product in separation zone 2 by means of at least one thermal separation process (or vice versa).
Thermal separation processes should be understood to mean those processes in which at least two substance phases different from one another (for example liquid/liquid; gaseous/liquid; solid/liquid; gaseous/solid, etc.) are generated and contacted with one another. Owing to the inequilibrium existing between the phases, there is heat and mass transfer between them, which ultimately causes the desired separation (removal). The term thermal separation processes reflects that either the withdrawal or the supply of heat is required to generate the formation of the substance phases and/or that the withdrawal or the supply of thermal energy promotes or maintains the mass transfer.
Thermal separation processes in the context of the present invention are therefore distillations, rectifications, crystallizations, extractions, azeotropic distillations, azeotropic rectifications, stripping, desorption, etc. (cf. also WO 04/063138).
Crystallizative thermal separation processes are considered to be particularly capital-intensive and attempts are therefore generally made to avoid them.